1. Field of the Invention
The present invention relates to an anti-reflection structure that is formed on an optical element as a substrate such as a lens.
2. Description of the Related Art
As a structure that is capable of preventing reflection of light on a surface of an optical element such as a lens and is a structure other than anti-reflection films using an interference effect obtained by a dielectric multilayer film, there is known a structure having roughness (convex and concave portions) in which a pitch of the convex portions is equal to or smaller than a wavelength of entering light. Such a structure is hereinafter referred to as a “minute roughness structure”. The minute roughness structure utilizes a property of light acting on convex portions whose pitch is equal to or smaller than a wavelength of the light as if the structure has a homogeneous effective refractive index.
Such a minute roughness structure is regarded as having an effective refractive index obtained by averaging a refractive index of a medium constituting the structure and a refractive index of air. Providing to the minute roughness structure such a structure that a medium density decreases from an interface (substrate side interface) between an optical element as a substrate and air toward the air, that is, providing refractive index gradient in which the effective refractive index gradually decreases from the substrate side interface toward the air prevents reflection of light at the substrate side interface.
However, securing physical or mechanical strength of the minute roughness structure is difficult, and a surface of the minute roughness structure cannot be wiped because it has roughness.
Japanese Patent Laid-Open No. 2008-9408 discloses a minute roughness structure that has an anti-reflection function and in which a porous layer as a low refractive index layer is formed in space (concave portions) around convex portions in order to increase mechanical strength of that minute roughness structure having a high aspect ratio. The porous layer has a significantly low refractive index of 1.05, so that the minute roughness structure including the porous layer also provides a good anti-reflection performance. Moreover, filling the space around the convex portions with the porous layer enables prevention of dust invasion into the space.
Furthermore, Japanese Patent Laid-Open No. 08-83581 discloses a thin film having an anti-reflection function utilizing refractive index gradient. Japanese Patent Laid-Open No. 08-83581 discloses that an anti-reflection layer having the refractive index gradient is provided on a surface of an electrically-conductive layer provided on a display surface of a display device in order to prevent static charge thereon and reduce electromagnetic leakage therefrom since a refractive index of the electrically-conductive layer is high.
However, in the minute roughness structure disclosed in Japanese Patent Laid-Open No. 2008-9408, since the space around the convex portions are filled with the porous layer whose refractive index is significantly low, a sufficient mechanical strength cannot be expected. A top surface of the minute roughness structure is formed almost only of the porous layer, so that the mechanical strength of the top surface is equivalent to that of the porous layer, which is low.
Moreover, the anti-reflection thin film disclosed in Japanese Patent Laid-Open No. 08-83581 has a sufficient mechanical strength, but does not have a sufficient anti-reflection performance. The anti-reflection layer provided in the anti-reflection thin film has a low anti-reflection performance only capable of reducing reflection at the electrically-conductive layer, and does not have a high anti-reflection performance capable of preventing reflection at a surface of an optical element.